1. Field of the Invention
This invention relates to improved catalytic systems, including MoVNbPd, MoVLaPd and combinations thereof, and to improved catalytic processes for the low temperature selective oxygenation of ethylene to acetic acid. More specifically, the invention relates to a single stage catalytic process using novel catalysts for providing high ethylene conversions and acetic acid yields.
2. Description of the Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
Acetic acid is generally produced by methanol carbonylation using an expensive rhodium catalyst in a liquid phase homogeneous reaction. This method requires complicated procedures for the recovery of the catalyst and the isolation of the products. Moreover, the presence of iodine at ppm levels in the final product has a negative impact on the usage of the acetic acid produced by the method.
Acetic acid is also produced by a two stage acetaldehyde process using manganese catalysts Such processes are disclosed in U.S. Pat. Nos. 3,131,223; 3,057,915; 3,301,905; and 3,240,805. The first stage of this process involves the production of acetaldehyde from ethylene. The economics of the process is not favored due to the costs arising from the two stages. Furthermore, these processes produce a large amount of acetaldehyde as a by-product. In addition, a large amount of ethylene may be lost by complete oxidation into carbon dioxide.
More recently, Showa Denko [EP 0 62 0205 Al] relates to a catalytic process for converting ethylene to acetic acid using catalysts containing heteropoly acids of phosphorus, silicon, boron, aluminum, germanium, titanium, zirconium, cerium, cobalt, chromium and metal palladium with at least one element selected from groups XI, XIV, XV, and XVI of the periodic table. The single pass conversion of ethylene was reported to be very low over these heteropoly catalysts and produces a significant amount of acetaldehyde, which can have a great impact on the separation cost. The catalytic systems used in the present invention are different from the Showa Denko catalysts.
EP A 0 29 4845 relates to a process for the higher selective production of acetic acid by the oxidation of ethane or ethylene with oxygen in contact with a physical mixture of at least two catalyst systems consisting of (A) a catalyst for oxydehydrogenation of ethane to ethylene and (B) a catalyst for hydration/oxidation of ethylene. The ethane oxydehydrogenation catalyst is represented by the formula Mo.sub.x V.sub.y Z.sub.z, wherein Z can be one or more of the metals Nb, Sb, Ta, Ca, Sr, Ti, W, Li, Na, Be, Mg, Zn, Cd, Hg, Sc, Fe and Ni. The catalyst for hydration/oxidation is selected from a molecular sieve catalyst, a palladium-containing oxide catalyst, tungsten-phosphorus oxide catalyst, or tin or molybdenum containing oxide catalyst. EP A 0 29 4845 employs the catalyst prepared by the physical mixing of the two types of catalysts.
Japanese Patent No. 46-6763 relates to the catalytic oxidation of ethylene to acetic acid using specific catalysts disclosed in the examples containing the following combination of metal atoms: V--Pd--Sb, V--Rh--Sb, V--Pd--P, V--Rh--P, V--Rh--As, V--Pd--As, Mo--Pd--Sb, Mo--Rh--Sb, Mo--Rh--As, and Mo--P--W--Pd--Rh--Sb. Japanese Patent No. 54-57488 relates to the use of NaPdH.sub.2 -PMoV catalysts for the oxidation of ethylene to acetic acid.
Syoji Tan et al. [J.Catal., vol. 17, pp. 132-142, 1970]reported that olefins oxidize to ketones over the binary catalyst systems Co.sub.3 O.sub.4 -MoO.sub.3 and SnO.sub.2 -MoO.sub.3. The article discloses the formation of acetic acid as a by-product together with other compounds and product of specifically ethylene was only carbon dioxide.
Thus, none of the prior art discloses or suggests the advantages of the catalytic system disclosed in present invention for the selective production of acetic acid from ethylene using a catalyst which is a dual function catalyst and is designed in such way that it enhances the activation function as well as the selectivity to the desired product acetic acid.